Eureka! Astronomers say they've discovered clear skies and water vapor on a Neptune-sized exoplanet named HAT-P-11b -- which, at roughly four times the Earth's radius, is the smallest planet yet known to host water in its atmosphere.
The discovery, described in the journal Nature, marks a milestone for scientists seeking to learn whether even smaller planets in far-off solar systems have atmospheres comparable to Earth's.
Finding exoplanets is difficult enough, but examining the molecular contents of their atmospheres is even harder. Scientists wait for the planets to pass in front of their stars and then examine the starlight that filters through the translucent gassy shell around the planet's silhouette. If there's water in that slim atmospheric lining, it will absorb certain wavelengths of the filtered light, leaving a chemical fingerprint that astronomers can identify.
It's a process known as transmission spectroscopy, and it's not easy work. Thus far, scientists have only been able to really probe the contents of the air around big planets, such as those the size of Jupiter, which are easier to spot and have wider, more extended atmospheres.
"Searching for water-vapor absorption in the atmosphere of an exoplanet passing in front [of] its host star is akin to looking for a tiny insect passing in front of a bright coastal lighthouse lamp," Eliza M.R. Kempton, an astronomer at Grinnell College in Iowa, wrote in a commentary on the study.
To make things worse, the atmospheres are often hidden by blankets of clouds. This is a problem familiar in our own solar system; until the 1970s, scientists knew very little about the surface of our neighbor planet Venus, because it was obscured by thick clouds of sulfuric acid, Kempton pointed out.
"We can infer the clouds exist because they're blocking all the signal," said lead author Jonathan Fraine, an astrophysicist and PhD student at the University of Maryland, College Park.
Fraine and his colleagues hit pay dirt when they checked out HAT-P-11b, a planet some 120 light-years away, in the constellation Cygnus, that circles its star roughly every 5 days. The transmission spectrum showed a clear signal from water vapor, as well as plenty of hydrogen. They cross-checked the surprising find with observations of NASA's Spitzer and Kepler space telescopes, to make sure the water wasn't coming from a sunspot backlighting the atmosphere.
The results held. And they seem to coincide with what planetary scientists expected, the authors wrote.
"The spectrum is indicative of a planetary atmosphere in which the abundance of heavy elements is no greater than about 700 times the solar value," the study authors wrote. "This is in good agreement with the core-accretion theory of planet formation, in which a gas giant planet acquires its atmosphere by accreting hydrogen-rich gas directly from the protoplanetary nebula onto a large rocky or icy core."
The fact that this Neptune-sized planet had clear skies gives scientists hope that even smaller planets will also have relatively cloud-free atmospheres, ripe for study.
"That's what exoplanets teach us," Fraine said. "Everything is unexpected."
Next up? A few more exo-Neptunes and perhaps even smaller, more ambitious targets – super-Earths, planets that have a few times the mass of Earth (and have no analog in our own solar system). The roster may even include planets in Earth's size range, Fraine said -- although they'd probably be very un-Earthlike, gassy and low in density.
Scientists will be able to better examine smaller planets' atmospheres with the 2018 launch of NASA's James Webb Space Telescope, Kempton wrote, because the giant scope has some instruments similar to Hubble's but is outfitted with a much larger mirror, which will allow it to pick up even fainter signals.